Achondrogenesis

Achondrogenesis: Description, Causes and Risk Factors: Neonatal lethal dwarfism characterized by severe bone dysplasia of all four limbs, micromelia, enlarged skull, and a short trunk with delayed or absent ossification of the lower spine and pubic bones. Achondrogenesis is a group of severe disorders that affect cartilage and bone development. These conditions are characterized by a small body, short limbs, and other skeletal abnormalities. As a result of serious health problems, infants with achondrogenesis usually die before birth, are stillborn, or die soon after birth from respiratory failure. Some infants, however, have lived for a short time with intensive medical support. Researchers have described at least three forms of achondrogenesis, designated as type 1A, type 1B, and type 2. The types are distinguished by their signs and symptoms, inheritance pattern, and genetic cause; however, types 1A and 1B are often hard to tell apart without genetic testing. achondrogenesis Type IA is an autosomal recessive disorder with an unknown chromosomal locus. In the current International Nomenclature of Constitutional Disorders of Bone, type IA is classified under spondylodysplastic and other perinatally lethal groups of osteochondrodysplasias. Type IB is an autosomal recessive disorder resulting from mutations of the diastrophic dysplasia sulfate transporter (DDST) gene (SLC26A2), which is located at 5q32-q33. Type II is an autosomal dominant type II collagenopathy resulting from mutations in the COL2A1 gene, which is located at 12q13.1-q13.3. Achondrogenesis is equally rare in males and females of all races in the United States. Although the exact incidence is unknown, one estimate places the incidence at 1 case in every 40,000 births. Symptoms: Common symptoms may include: Very short trunk, arms, legs and neck.
  • Head appears large in relation to the trunk.
  • Small lower jaw.
  • Narrow chest.
  • Protuberant abdomen.
Diagnosis: Prenatal diagnosis of a skeletal disorder may be made by ultrasound. DNA testing may be used to determine the type of disorder, or to confirm the presence of a suspected disorder. Otherwise, diagnosis may be made by the physical appearance of the infant at birth, and/or x rays. DNA analysis or a microscopic examination of cartilage tissues may be used to identify the type of disorder. Molecular studies for achondrogenesis are performed on ethylenediaminetetraacetic acid (EDTA)-anticoagulated blood for DNA analysis.
  • Mutation analysis of the DDST gene identifies the following: point mutations, deletions leading to premature stop codons, substitutions or deletions of amino acids within transmembrane domains, substitutions of amino acids in intracellular or extracellular domains, and presumed mutations lying outside the coding region but causing low mRNA levels.Several mutations of the DDST gene have been reported in patients with type IB (the most severe form), patients with atelosteogenesis type II (an intermediate form), and patients with diastrophic dysplasia (the mildest form).Mutation analysis can be used to ascertain carriers, particularly in consanguineous families. However, biochemical analysis of fibroblast cultures has not been able to distinguish heterozygotes from normal homozygotes.
  • Mutation analysis of the COL2A1 gene detects a single base change that has been observed in a patient with achondrogenesis type II in the type II procollagen gene (ie, substitution of serine for glycine in the alpha 1 [II] chain).
Imaging: Radiological features may vary, and no single feature is obligatory. Distinction between type IA and type IB on radiographs is not always possible. Degree of ossification is age dependent, and caution is needed when comparing radiographs at different gestational ages. Treatment: There is no known treatment for the underlying disorder. Parents should consider mental health and genetic counseling to deal with the grief of losing a child, and to understand the risks of the disorder recurring in subsequent children. Support groups may be helpful in the pursuit of these goals. It is important for genetic counseling purposes to determine the type of achondrogenesis that affected the child, since different types of achondrogenesis carry very different prognoses for future children. NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.

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